Wireless communication systems are well known in the art. Generally, such systems comprise communication stations, which transmit and receive wireless communication signals between each other. Depending upon the type of system, communication stations typically are one of two types: base stations or wireless transmit/receive units (WTRUs), which include mobile units.
The term base station as used herein includes, but is not limited to, a base station, a Node B, a site controller, an access point (AP), or other interfacing device in a wireless environment that provides WTRUs with wireless access to a network with which the base station is associated. In a WLAN, an AP is the preferred device, and will be used herein to describe the present invention.
The term WTRU as used herein includes, but is not limited to, a user equipment, a mobile station, a fixed or mobile subscriber unit, a pager, or any other type of device capable of operating in a wireless environment. WTRUs include personal communication devices, such as phones, video phones, and Internet ready phones that have network connections. In addition, WTRUs include portable personal computing devices, such as PDAs and notebook computers with wireless modems that have similar network capabilities. WTRUs that are portable or can otherwise change location are referred to as mobile units. In a WLAN, a station (STA) is the preferred device, and will be used herein to describe the present invention.
A popular WLAN environment with one or more APs is built according to one of the IEEE 802.11 family of standards. The basic service set (BSS) is the basic building block of an 802.11 WLAN and consists of a plurality of STAs. A set of STAs which can talk to each other can form a BSS. Multiple BSSs are interconnected through an architectural component, called a distribution system (DS), to form an extended service set (ESS). An AP is a STA that provides access to the DS by providing DS services and generally allows concurrent access to the DS by multiple STAs.
Systems compliant with the 802.11a/b/g WLAN standards are in widespread use. Quality of Service (QoS) capable WLANs are also emerging. The QoS solutions/specifications vary between proprietary solutions and standardized solutions where even the standard solutions have two types of QoS schemes, namely: 802.11e enhanced distribution coordination function (EDCA) and 802.11e hybrid coordination function (HCCA) schemes.
The 802.11e QoS facility defines a new hybrid coordination function (HCF) used only in a QoS enhanced basic service set (QBSS). HCF has two modes of operation: HCF with contention operation (also known as EDCA) and HCF with polled access operation.
A basic concept utilized by these channel access functions is the transmission opportunity (TXOP). A TXOP is a point in time when a STA can begin transmitting frames for a given duration. During a TXOP, a STA can transmit as many frames as possible in the TXOP, which is set according to the traffic class (TC) associated with the data.
A key difference between EDCA and HCCA is in how admission control is handled. In EDCA, admission control is handled by a distributed admission control (DAC) function, which measures the utilization percentage of the medium for each TC. The unused percentage of the medium is known as the available transmission budget for the TC. As the transmission budget approaches zero, limitations are imposed on adding new traffic flows or modifying existing traffic flows.
In HCCA, a STA requests particular reservation parameters for each traffic flow, based on the TC of the traffic flow. The HCF can then reject, accept, or offer an alternative set of parameters to the requesting STA. While this is more robust than DAC, the traffic flows need to be properly scheduled.